The objective of this proposed research is to establish structural reasons for the general hormonal activity of thyroactive compounds and for the explicit thyroidal-endocrine mechanisms which mediate neoplastic processes. Although chemical observations suggest that thyroactivity is a function of the specific orientation of the outer phenyl ring with respect to the inner ring and the specific positions occupied by acceptable sutstituents, the precise geometric description of the compounds, such as the distances between functional groups, their angular relationships, or the actual size and shape of the molecules were not known until recently. Such information can only be obtained from crystal and molecular structure determination. Therefore it is proposed to determine the exact crystal and molecular structures of a series of thyroactive compounds employing X-ray crystallographic techniques and conformational energy studies. The molecular architectures of the compounds will be examined for structural features which are associated with thyroactivity.